1. Field of the Invention
The present invention relates to a method of recording and reading information using an optical phase, a method of evaluating a read signal, and an information recording and reading apparatus for implementing the methods.
2. Description of the Related Art
The following description includes some terms provided using expressions used for a Blu-ray Disc (BD). Other terms may be used in the case of systems other than a BD system. However, it is easy for those skilled in the art to read the terms for the BD as the terms in the other systems.
Increase in storage capacity of an optical disc has been achieved by increasing the number of recording layers per disc in addition to using an optical source having a shorter wavelength and increasing a numerical aperture (NA) of an objective lens. A dual layer BD using a blue semiconductor laser and a high-NA objective lens having an NA of 0.85 achieves a storage capacity of 50 GB. Moreover, a BD XL having has been put to practical use in 2010. The BD XL achieves a storage capacity of 100 GB or higher by increasing the number of recording layers to 3 or 4 and enhancing the surface recording density at the same time.
However, the reduction in the recording wavelength and the increase in the NA of the objective lens are almost reaching the limits and it is not easy to drastically increase a surface recording capacity. Therefore, one of potent solutions for increasing the recording capacity over the above described level is to further increase the number of recording layers. With use of the same configuration as a conventional multilayer optical disc, however, a simple increase in the number of recording layers, even if achieved, is highly likely to fail reduction in cost per storage capacity. This is because manufacturing costs and yields of currently available multilayer optical discs are mainly determined by formation processes of recording layers. That is to say, an increase in the number of layers is directly linked to an increase in the number of processes and a final yield is basically determined by a value of a yield of a stamping process per layer raised to the power of the number of layers.
To address this, studies are in process on an optical disc not including physically defined recording layers unlike a conventional multilayer disc and a recording technique for such an optical disc. In one example, Patent Document 1 discloses a technique for recording a micro-hologram, i.e., a minute interference fringe, within interior of a recording region made of a photorefractive material. Since there are no structures for physically defining recording positions in this recording region, a recording position of each micro-hologram is determined by indirectly controlling a focal position of light used for recording (recording light). In another example, there is also a technique to perform recording by forming voids in a recording region as described in Patent Document 2. According to these recording methods, it is possible to increase virtual recording layers relatively freely and to increase the recording capacity per disc easily. It is to be noted that the above-described methods using no layer for physically defining the recording positions in the recording regions will be generally referred to as volume recording in this specification for convenience sake.
Reduction in the amount of reflecting light from a layer used for reading is a problem in the cases of increasing the number of recording layers including the above-described case of volume recording. Since output of a recording light source is limited, a disc provided with multiple recording layers needs to include intermediate recording layers each having sufficiently high transmissivity in order to record on the layer farthest from an entrance plane of reading light. In other words, optical reflectivity and absorptance of each of the layers needs to be sufficiently small. Meanwhile, since a recording film is set to have high recording sensitivity for recording on the recording layer having small absorptance, there is a limit to increase power of an outgoing beam (reading light) from a pick-up at the time of reading. For this reason, the amount of light returning from a recording layer at the time of reading generally becomes smaller with an increase in the number of recording layers. Accordingly, there is a problem of reduction in a signal to noise ratio (SNR) of a read signal.
As a technique for coping with the reduction in the SNR of the read signal, there is a signal amplitude amplification technique based on application of optical interference as disclosed in Patent Document 3. Specifically, this technique amplifies the read signal by causing reference light (beam) obtained from a light source which also emits reading light to interfere with reflecting light of the reading light from a recording layer on an optical detector. It is to be noted that the method of causing the reference light obtained from the light source which also emits the reading light to interfere with the reading light on the optical detector and a reading optical system for that purpose will be respectively referred to as a homodyne detector and a homodyne detection system in this specification.
One of key performances of an optical disc drive is a data transfer rate (hereinafter simply referred to as a transfer rate) at the time of recording and reading. This is an important performance factor particularly in the case of non-consumer use. The transfer rate is primarily determined by a linear recording density and a linear speed of a disc. Meanwhile, the linear speed of the disc is limited by an achievable revolution speed of the disc. In the case of a disc having a diameter of 12 cm and being made of polycarbonate, which is used in almost all discs, the revolution speed of the disc is estimated to have a limit around 10000 rpm (rotations per minute) considering vibration and deformation.
The linear recording density is primarily determined by optical resolution of a reading head, and is determined further in consideration of a practical performance margin as well as a performance increasing effect attributed to signal processing. The optical resolution is determined by a wavelength of a light source used by the head and an opening ratio of an objective lens. Specifically, an upper limit for the transfer rate of the optical disc drive is mainly determined by the limit of the achievable revolution speed of the disc and by the linear recording density. The above-mentioned matters are known to those skilled in the art and further detailed description will therefore be omitted herein.
Nevertheless, the optical resolution is almost reaching the limit as described previously. A multilevel recording method for recording information exceeding 1 bit per channel clock is a method highly expected to further improve the transfer rate in this situation instead of conventional binary recording of recording 1 bit per channel clock. The multilevel recording method also increases a storage capacity per unit length, which naturally leads to an increase in the storage capacity of the disc.
As the multilevel recording method, there is a technique disclosed in Non-patent Document 1. In this technique, a recording waveform is improved to increase the number of levels in modulation of the reflectivity of a recording film of a kind similar to in a conventional optical disc from two levels to eight levels, and thereby the storage capacity is increased. However, since the amplitude of the read signal is the same as that in the conventional optical disc, the SNRs at signal levels corresponding to the respective levels of reflectivity are reduced. Accordingly, this technique is not always suitable for improving the transfer rate.